1. Filed of the Invention
The present invention relates to a rubber composition, especially a rubber composition for a tire tread containing a modified polyisobutylene rubber having an improved grip performance without substantially decreasing the abrasion resistance and the rolling resistance.
2. Description of the Related Art
Since butyl rubber, one of the polyisobutylene rubbers, has a high tan .delta. at 0.degree. C., an improvement in the grip performance is expected. However, since the reinforcability with carbon black is poor, when used as a tire tread, the abrasion resistance is decreased. Further, since the tan .delta. at 60.degree. C. is increased, the rolling resistance is deteriorated. Therefore, the butyl rubber is not substantially used for tire treads.
Japanese Unexamined Patent Publication (Kokai) No. 63-270751 discloses a technique for improving the grip of a tire tread by adding a specific carbon black to a rubber blend selected from at least 20% styrene-butadiene copolymer rubber, halogenated butyl rubber, etc. and also discloses that good results are obtained when silica is added, however, nothing is taught, regarding the synergistic effect of the combination of the three elements of a styrene-butadiene copolymer rubber, a halogenated butyl rubber, and silica. Further, no examples of the same are disclosed either. Further, Japanese Unexamined Patent Publication (Kokai) No. 4-227741, Japanese Unexamined Patent Publication (Kokai) No. 6-55040, etc. discloses silica-reinforced halogenated butyl rubbers, but the rubbers blended are natural rubbers and the effect thereof in the decrease of the tan .delta. at 60.degree. C. is not taught. Further, Japanese Unexamined Patent Publication (Kokai) No. 7-304903 discloses a technique for improving the friction and the rolling resistance and abrasion resistance of tires by adding a specific halogenated isobutylene rubber to a silica-containing isoprene-butadiene copolymer rubber, but this publication teaches nothing about the conditions of the tan .delta. at 60.degree. C. of 0.2 or more. Therefore, in examples where a halogenated polyisobutylene is added, the tan .delta. at 60.degree. C. increases and the rolling resistance is decreased compared with the control formulation.
As explained above, polyisobutylene rubbers such as halogenated butyl rubber have a high tan .delta. at 0.degree. C., which is correlated with the frictional force, when compared with diene rubbers such as natural rubber or styrene-butadiene copolymer rubber, and therefore, is effective for improving the grip performance when used for a tire tread. However, a polyisobutylene rubbers have the defects of being poor in bondability with carbon black, and therefore, reduction in the abrasion resistance and have the defects of being poor in the rolling resistance due to the high tan .delta. at 60.degree. C., and therefore, has not been substantially used for actual tire treads.